1. Field of the Invention
This invention relates to semiconductor thyristors and, in particular, to semiconductor thyristors having a recombination region therein.
2. Description of the Prior Art
Nearly all large, high power thyristors are made with shunted cathodes to provide for high dv/dt and suitable short turn-off time capabilities. These shunts typically consist of small, localized circular or line-shaped regions in which the cathode metal electrode is shorted to the p-base that under lies the cathode. Their function in the conventional-type thyristor is to decrease the low-current gain of the npn transistor portion so that displacement currents caused by applied dv/dt do not fire the thyristor. Under proper operating conditions, shunting action prevents electron emission from the cathode and the thyristor can continue to block forward voltage.
Discrete shunts take up little of the conducting area when the thyristor is in its normal conducting (high current) state. On the other hand, a thyristor being unintentionally switched by dv/dt-induced current is in a state in which only small areas of the emitter (i.e. those areas farthest from the shunts) carry current, causing a high current density or current crowding in these areas. This concentration of the cathode current increases the emitter efficiency and increases the likelihood that the thyristor will fire prematurely.